486 STRUCTURE OF VERTEBRATA. 
and the parietal organ show an eye-like structure, most marked in the 
case of the epiphysis. 
Fic. 259.—Vertical section of the 
pineal eye in an embryo of Spheno- 
don.—After Dendy. 
E., Epidermis; D., dermis; Z., lens; /.W., 
inner wall of the eye; O.W. outer wall of 
the eye; PA.N., parietal nerve; PA.S., 
parietal stalk; C., cartilage. 
of the parietal organ) receives a 
nerve from a ‘‘ parietal centre” 
near the base, but independent 
of the epiphysis; this nerve is 
transitory in Amguts, more or 
less persistent in /gwana. Above 
Reptiles the pineal stalk is relatively 
short, and its terminal portion is 
glandular. Among mammals the 
epiphysis is absent in the dugong 
and some Cetaceans; the pineal 
body is absent in Dasypus and the 
dolphin. 
The significance of the pineal 
body is uncertain. According to 
some, its primitive function is that 
of an unpaired, median, upward- 
looking eye—a function retained 
only in the Reptiles mentioned 
above, the organ having elsewhere 
undergone (independent) degenera- 
tion. It may be, however, that the 
optic function is not primitive, but 
the result of a secondary transforma- 
tion, 
In Elasmobranchs the pineal 
process (epiphysis) is very 
long, and, perforating the 
skull, terminates below the 
skin in 4 closed vesicle. In 
the young frog it also comes 
to the surface above the skull, 
but degenerates in adoles- 
‘cence. In Sphenodon the 
stalk passes through the skull 
by the ‘parietal foramen,” 
so that the ‘‘eye” itself, 
developed from the parietal 
organ, lies close beneath the 
skin, the scales of which 
in this region are specialised 
and transparent. In Jgwana, 
Anguis, Lacerta, etc., the 
epiphysis loses connection 
with the ‘‘eye” portion; 
and it is also to be noticed 
that in Amguzs and Jguana 
the pineal body (on the end 
Fic. 260.—Diagram of the parts 
of the brain in Vertebrates. — 
After Gaskell. 
c.4., Cerebral hemispheres; c.f2., 
choroid pisses; o.th., optic thal- 
ami; 0.4, optic lobes; ¢é., cere- 
bellum; ¢.f2, choroid plexus; 
4M.0., medulla oblongata; S.C., 
spinal cord. 
